FIG. 1 illustrates an image capture system, such as a camera, having an imaging lens 10 with a focal length f and a detector 12. An object 14 at a distance d.sub.o in front of the camera will be properly focused at the detector 12 at a distance d.sub.i behind the lens if the relationship between d.sub.o, d.sub.i, and f is ##EQU1##
If the camera-to-object distance changes to a distance d.sub.o +.delta..sub.0 in front of the camera, as illustrated in FIG. 2, then the image will be out of focus by a distance .delta..sub.i given by ##EQU2## No adverse effect to the image quality will be perceived if .delta..sub.i is within the depth of focus of the image capture system, given by ##EQU3## where D is the entrance aperture diameter of the image capture system. The corresponding depth of field .delta.'.sub.o is given by ##EQU4## hence objects within the range of d.sub.o +.delta..sub.o will not appear out of focus.
The correct focus of an image capture system can change over time due to dynamic changes with the system, such as those caused by the thermal environment. When the camera-to-object distance changes more than .delta.'.sub.o it is necessary to refocus the image capture system. Prior art methods and devices have been proposed for determining the system focus by evaluation of the sharpness of an image formed by the system. These scene-based methods generally compare the image sharpness of the same scene imaged at different focus positions. A challenge with scene-based autofocus methods is that it is difficult to separate changes in focus from normal changes in the scene. This problem can be solved by imaging the same scene multiple times at different focus settings to determine the best focus. One such method proposed in U.S. Pat. No. 5,166,506 uses beam splitters to image the scene onto a plurality of sensors. This method, however, requires additional optical elements and the beam splitters reduce the signal arriving at each sensor. Another method proposed in U.S. Pat. No. 5,432,331 uses a tilted array and the image is moved such that the same scene is imaged onto each line of the sensor. This method requires very precise scanning to ensure that the same scene is properly imaged onto each line of the sensor.
There is a need therefore for a scene-based autofocus method that does not require the same scene to be imaged multiple times to determine focus and does not require additional optical elements in the design of the image capture system.